Hard disk drives are used in almost all computer system operations, and recently even in consumer electronic devices such as digital cameras, video recorders, and audio (MP3) players. In fact, most computing systems are not operational without some type of hard disk drive to store the most basic computing information such as the boot operation, the operating system, the applications, and the like. In general, the hard disk drive is a device which may or may not be removable, but without which the computing system will generally not operate.
The basic hard disk drive model was established approximately 50 years ago. The hard drive model includes a plurality of storage disks or hard disks vertically aligned about a central core that can spin at a wide range of standard rotational speeds depending on the computing application in which the hard disk drive is being used. A plurality of magnetic read/write transducer heads, where a head reads data from and writes data to a surface of a disk, are mounted on actuator arms.
Data is formatted as written magnetic transitions (information bits) on data tracks evenly spaced at known intervals across the disk. An actuator arm is utilized to reach out over the disk to or from a location on the disk where information is stored. The complete assembly at the extreme of the actuator arm, e.g., the suspension and magnetic read/write transducer head, is known as a head gimbal assembly (HGA).
In operation, the hard disks are rotated at a set speed via a spindle motor assembly having a central drive hub. When a request for a read of a specific portion or track is received, the hard disk drive aligns a head, via the actuator arm, over the specific track location and the head reads the information from the disk. In the same manner, when a request for a write of a specific portion or track is received, the hard disk drive aligns a head, via the actuator arm, over the specific track location and the head writes the information to the disk.
Over the years, refinements of the magnetic recording technology for the disk and head have provided the capability to increase, by many orders of magnitude, the amount of magnetic data information that can be recorded per unit of disk surface area. This in turn has led to substantial reductions in the size of the hard disk drive. For example, an original hard disk drive had many disks, each of which had a diameter of 24 inches. The disk platter diameter has, over time, evolved downward to 356 millimeters, 200 millimeters, 130 millimeters, and 95 millimeters. Present day hard disk drives have fewer disks, are generally much smaller, and the disks may have a diameter of 95 millimeters, 65 millimeters, 48 millimeters, or smaller. Recently developed miniature hard disk drives (MHDD) have disk diameters of 27.4 millimeters or smaller.
An additional refinement to the hard disk drive, resulting from the reduction in disk diameter size and the corresponding reduction in air drag or power associated with rotation of the disk platter, is the increased efficiency and reduced size of the spindle motor spinning the disk. When the diameter of the disk became smaller than 200 millimeters, it became common design practice to rotate the spindle, for those size disks, with a brushless DC motor directly coupled to the central drive hub, with the motor packaged within the hub.
Prior art FIGS. 5D and 5E are side elevation views of two such spindle motors, and those skilled in the art readily comprehend how there is sufficient radial space within the disk platter hole to package the central drive hub, stator components, rotor components, and bearing components to enable rotation of the assembly about that bearing system. Prior art FIGS. 5D and 5E illustrate some of the components of a spindle motor disposed in the platter hole and which can include, but which are not limited to, a shaft 37, a bearing 37b, a back iron 31, a radially poled magnet rotor component 32, a coil 24 wound around a stator stem 22 and a screw 95 and a clamp 96 for retaining a magnetic disk(s) 15.
As technology has reduced disk size and power draw for small motors, the mechanical portion of the hard disk drive can be reduced and additional revolutions per minute (RPM) of the spindle can be achieved. For example, it is not uncommon for a hard disk drive disk having a diameter of 65 millimeters to reach speeds of 15,000 RPM. Increased rates of revolution provide a faster read and write rate for the disk and decrease latency, the time required for a data area to become located beneath a head, thereby increasing data access speed. The increase in data access speed due to the increased RPM of the disk drive, and the more efficient read/write head portion, provide modem computers with data access performance and storage capabilities that are continually increasing for those applications that require performance. In other applications such as mobile laptop computers, the hard disk drive, which typically now has only one or two 65 millimeter disks, is rotated at a much lower range of RPMs (e.g., from approximately 4200 to approximately 7200 RPMs), and efficient storage and transfer of data is effected at power levels consistent with use of a battery for the computer power source.
Particularly, with regard to the continued miniaturization of the hard disk drive, smaller diameter disk platters having a hole of limited size has led to the development of diminutively sized spindle motors. These miniature hard disk drives (MHDD) typically have only one disk platter and a very low profile height of approximately 5.0 millimeters. MHDDs having low form factor height have limited cylindrical volume (diameter times height) for packaging the spindle motor stator and rotor, but still require a high spindle motor torque constant for high start torque, low run current, and therefore low operational power requirements.
Minimizing necessary operational power is a key attribute required in MHDDs and particularly beneficial for mobile applications. In MHDDs, the entire form factor height of the MHDD is effectively utilized by the spindle motor package. Further, because of height constraints, the card/printed circuit board (PCB) for the MHDD electronics requires one or more openings therein to accommodate the spindle motor and related packaging. In MHDDs, this/these hole(s) can occupy a substantial portion of the card (PCB) such that there can be insufficient space to locate the necessary electronic components, especially if one of those components is a large area highly integrated electronics package.
FIG. 5A (prior art) is an illustration of a conventional low profile hard disk drive 10 currently available, shown with the spindle bearing and hub removed, and only the outside perimeter of the disk defined. A side elevation of the spindle of drive 10 of FIG. 5A is shown in FIG. 5C. Drive 10 includes one or more hard disks 15 from which data is read or to which data is written. Drive 10 includes, in part, an actuator arm 4 having coupled thereto a read/write head for reading from and writing to disk 15. Also shown in drive 10 is a typical spindle motor 20 having a stator/coil unit 21 and a rotor unit 30 with radially poled permanent magnets that work in cooperation to create an electromagnetic force at the torque radius to rotate the spindle and disk. As the stator/coil unit and its rotor are disposed beneath a portion of the disk surface area that the actuator and HGA with the read/write transducer head must traverse (as indicated by arrow 36), there is a severe height constraint in this region for the allowable height of these torque generating motor components.
Hard disk 15 of drive 10 typically has a radius R1 of approximately 13.7 millimeters and an inside hole diameter of approximately 7.0 millimeters. Spindle motor 20 typically has a stator/coil unit 21 having nine slots (nine stems) and a permanent magnet rotor 30 with twelve poles. Typically, spindle motor 20 generates a maximum constant torque output of less than 2.9 Newton millimeters per ampere, that value being a threshold desired for 3.3 volt power supplies, given the drag load associated with this size spindle bearing and disk diameter.
FIG. 5B (prior art) is an illustration of the stator/coil unit 21 and permanent magnet rotor 30 of a typical spindle motor 20 shown removed from drive 10 of FIG. 5A. Spindle motor 20, in the shown configuration, has a stator with an outer diameter OD1 of approximately 17.5 millimeters and an inside diameter ID1 of approximately 9.7 millimeters. Spindle motor 20 includes an inner rotor unit 30 having a back iron member 31 and a radially polarized permanent magnet 32 consisting of twelve poles. Spindle motor 20 further includes an outer stator/coil unit 21 having nine stator stems 22, and around each stator stem 22 is a coil winding 24. Further, each stem 22 of stator unit 21 terminates at the tip 28 in a geometric shape termed a tooth or pole tip shoe that is configured to be proximal to the outer edge of the magnetic poles of rotor 30 to efficiently capture the magnetic flux there from. Stator unit 21 is shown to have a three phase coil winding, e.g., phases 40, 50 and 60. Each phase 40, 50, and 60, has associated therewith three stems 22, e.g., stems 41, 42, 43; 51, 52, 53; 61, 62, and 63, respectively.
It is particularly noted that stator 21 is configured such that each stem 22 of each phase 40, 50, and 60 is located plus or minus 120 degrees from another stem 22 of each phase. This severely limits any attempts to modify a winding 24 about a stator stem 22 without each winding being similarly modified. Because there is no stem directly opposite (180 degrees) another stem, all stems in stator 21 must be modified analogously. If non-analogous modifications are performed, a radial force unbalance may be introduced when, for the purpose of rotating the spindle, the plurality of coil windings of each phase are sequentially energized to provide conductivity in combination with the magnetic flux from the corresponding poles of the rotor to create an electromagnetic force and rotation torque, at the radius between the outer diameter of the rotor magnet and inner diameter (ID1) of the stator.
Also shown in FIG. 5B is an opening 29 through which a manufacturing winding needle must pass to create each coil winding 24 that is wound around each stem 22. Opening 29 between the stem teeth of stator 21 can range from approximately 1.25 millimeters to 1.12 millimeters, or effectively less if the height or thickness of the coil exceeds the boundary of the stem tooth and thereby reduces the available access. This means that, in addition to the dimension of the opening 29, the capability to wind the stem 22 can also be a function of the coil thickness 77.
Cross-section 97 is an illustrated cross-section taken through a stator stem 22, e.g., stem 62, and its coil winding 24 of motor 20 of FIG. 6B. Stator stem 22 of cross-section 97 is comprised of four lamination layers 25, has a width 71 of approximately 1.3 millimeters and a height 72 of approximately 0.8 millimeters, which provides a stem cross-sectional area of approximately 1.04 square millimeters. This area is sufficient to carry the flux from the poles of the rotor magnets. The outside of the lamination stack, including the stem, is coated with an insulating coating approximately 25 to 50 microns thick so the coil wire is not damaged when it is wound around the stem. Wound around each stem stator 22 is a winding 24 which typically includes six layers of 0.080 millimeter diameter wire. This results in approximately 118 turns per stem equating to 354 turns per phase, and results in a total coil height 76 ranging from approximately 1.88 millimeters to approximately 2.06 millimeters and a total coil width 75 ranging from approximately 2.33 millimeters to approximately 2.51 millimeters for a wound stem 22. The resulting high resistance phase to phase winding, in co-operation with the magnetic flux from the rotor magnet poles, creates a motor torque constant of approximately 2.9 Newton millimeters per ampere.
Those skilled in the art will readily understand for the stator/coil unit 21 and rotor 30 shown in the prior art configuration of FIG. 5B, if the OD1 dimension is reduced and thus the available radial length 78 for the coil winding is reduced, then at a total stator/coil unit height 76 dimension constraint and a phase to phase resistance constraint, fewer turns per phase can be realized, therefore degrading the motor torque constant.
Present spindle motors 20, if reduced in size, may not be able to provide proper operational functionality. By reducing the size (diameter and height) of the spindle motor 20 to comply with other requirements such as electronics card area of the diminutive form factor of miniature hard disk drives, the functionality of spindle motor is reduced. Further, a reduced sized spindle motor 20 may not be able to provide sufficient constant motor torque given the available operational power, the usable real estate within the printed circuit board, and the physical limitations and height constraints inherent of a miniature hard disk drive. Therefore a new stator/coil unit and rotor configuration is needed for the spindle motor for implementation in these low profile hard disk drives of diminutive size.